(a) Field of the Invention
The present invention relates to an electronic apparatus for measuring the difference of elevation between two points at different locations. More particularly, the invention relates to an electronic apparatus for measuring such a difference of elevation in an automatic manner and with a compensation of any variation of temperature, and for immediately displaying the so-measured value.
(b) Description of the Prior Art
A conventional method for measuring the difference of elevation between two points or the height of a point with respect to another, consists in measuring the length and the angle of inclination of a line joining these two points with an optical apparatus such as a stadia or a theolite and thereafter calculating the desired value from the so-measured data.
This conventional method while being technically accurate per se, is however time consuming and requests a specialized staff. Moreover, this conventional method is carried out with substantial difficulties or cannot even be carried out at all when the measurement has to be made in a wooded or broken area where there is no visual contact between the two points between which the difference of elevation has to be measured. To overcome this drawback specific to the conventional method, non-optical apparatuses have been proposed. Examples of such non-optical apparatuses are described in U.S. Pat. No. 2,557,021 to Williams issued on June 12, 1951; U.S. Pat. No. 2,851,799 to Meents et al. issued on Sept. 16, 1958; U.S. Pat. No. 3,494,202 to Komay issued on Feb. 10, 1970; U.S. Pat. No. 3,815,423 to Gearhart issued on June 11, 1974 and U.S. Pat. No. 2,844,037 to Jordan issued on July 22, 1958.
Generally, all these known apparatuses can be used in geodesical survey for determining the vertical distance between two points located at two different levels by measurement of a hydrostatic pressure difference. The measurement of this hydrostatic pressure difference and the reading of this measurement however is made by mechanical means, such as, for example, by movement of a piston or a flexible membrane caused by the hydrostatic pressure. If these known apparatuses have some advantages, they also have substantial drawbacks. In particular, these known apparatuses have a small accuracy and they are restricted in use and application because of their size, when the difference of elevation to be measured is of a substantial value.
U.S. Pat. No. 4,145,817 to Ager issued on Mar. 27, 1979 describes an apparatus comprising a pair of electronic pressure transducers located at the extremities of a tube filled with a fluid. The electronic signals generated by the transducers as a function of the fluid pressures at both extremities of the tube are transmitted to an electronical circuit which evaluates the difference of elevation between the two transducers. This type of apparatus is a little more accurate than the above mentioned known apparatuses. However, the data displayed by this apparatus are also subject to error due to the difference of temperature of the fluid inside tube. Indeed, if one extremity or one part of the tube is exposed to the sun and the other is in the snow, the temperature of the fluid inside the tube varies from one extremity thereof to the other and generates a measuring error of about 0.13% per .degree.C. In some cases, the so generated error can be substantially higher than the maximum allowance that is necessary in some applications. Thus, the apparatus disclosed in this patent cannot really be used in areas where part of the tube passes under the shadow of trees and other parts of the same tube are exposed to the sun.